081 effect branch labeled loop

// Test: labeled-loop re-invocation around an effect-bearing event.
//
// `ramp(start: x)` is a pump: it emits `! step i` for two iterations,
// then signals `| restart (start+2)` or `| done` based on whether to
// continue. The consumer re-invokes the producer with NEW args via
// `@loop(start: n)` — this is the labeled-loop re-entry shape.
//
// The producer's INTERNAL loop is invisible to the consumer (effect
// branches handle that). The consumer's EXTERNAL loop is the labeled
// re-invocation. Both layers compose: each call to the producer fires
// its effects, then the consumer either restarts or completes.
//
// This proves that pre_label handler injection (the wiring that was
// landed but never exercised) survives an end-to-end run when the
// labeled event has BOTH effect branches AND a re-entering terminal.

~import std/io

~pub event ramp { start: usize }
! step usize
| restart usize
| done usize

~proc ramp|zig {
    var i: usize = start;
    while (i < start + 2) : (i += 1) {
        step(i);
    }
    if (start < 4) {
        return .{ .restart = start + 2 };
    }
    return .{ .done = i };
}

~#loop ramp(start: 0)
! step v |> std/io:print.blk {
    step {{ v:d }}
}
| restart n |> @loop(start: n)
| done total |> std/io:print.blk {
    done {{ total:d }}
}